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Re: [RFC][AArch64] function prologue analyzer in linux kernel


On 01/07/2016 11:56 PM, Richard Earnshaw (lists) wrote:
On 07/01/16 14:22, Will Deacon wrote:
Akashi,

On Thu, Dec 24, 2015 at 04:57:54PM +0900, AKASHI Takahiro wrote:
I'm the author of ftrace support on arm64(aarch64) linux. As part of
ftrace, we can utilize "stack tracer" which reports the maximum usage
of kernel stack:

We're probably missing some important background here -- I suspect most
of the GCC folk are wondering wtf this is and what it's doing on their
list ;)

It's partly my fault, since I asked you to run this by the compiler guys,
but that's because I have concerns on the approach. See below.

---8<---
# cat /sys/kernel/debug/tracing/stack_max_size
4088
# cat /sys/kernel/debug/tracing/stack_trace
         Depth    Size   Location    (49 entries)
         -----    ----   --------
   0)     4088      16   __local_bh_enable_ip+0x18/0xd8
   1)     4072      32   _raw_read_unlock_bh+0x38/0x48
   2)     4040      32   xs_udp_write_space+0x44/0x50
   3)     4008      32   sock_wfree+0x88/0x90
   4)     3976      32   skb_release_head_state+0x70/0xa0
  [snip]
  44)      808      32   load_elf_binary+0x29c/0x10d0
  45)      776     224   search_binary_handler+0xbc/0x208
  46)      552      96   do_execveat_common.isra.15+0x4e4/0x690
  47)      456     112   SyS_execve+0x4c/0x60
  48)      344     344   el0_svc_naked+0x24/0x28
--->8---

Here, "Depth" (and hence "Size") is determined, after scanning a stack,
by saved fp pointer (more precisely + 0x10) in a stack frame instead
of (not saved) stack pointer. (Please note that arm64 kernel is always
compiled with -fno-omit-frame-pointer.)

As fp is updated after branching into a function, and allocates not only
a function's stack frame but also callee's local variables, using this
saved value of fp as "Depth", or sp of a caller function, is not
appropriate for calculating a stack size of a function.

So I'd like to introduce a function prologue analyzer to determine
a size allocated by a function's prologue and deduce it from "Depth".
My implementation of this analyzer has been submitted to
linux-arm-kernel mailing list[1].
I borrowed some ideas from gdb's analyzer[2], especially a loop of
instruction decoding as well as stop of decoding at exiting a basic block,
but implemented my own simplified one because gdb version seems to do
a bit more than what we expect here.
Anyhow, since it is somewhat heuristic (and may not be maintainable for
a long term), could you review it from a broader viewpoint of toolchain,
please?

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/393721.html
[2] aarch64_analyze_prologue() in gdb/aarch64-tdep.c

My main issue with this is that we cannot rely on the frame layout
generated by the compiler and there's little point in asking for
commitment here. Therefore, the heuristics will need updating as and
when we identify new frames that we can't handle. That's pretty fragile
and puts us on the back foot when faced with newer compilers. This might
be sustainable if we don't expect to encounter much variation, but even
that would require some sort of "buy-in" from the various toolchain
communities.

GCC already has an option (-fstack-usage) to determine the stack usage
on a per-function basis and produce a report at build time. Why can't
we use that to provide the information we need, rather than attempt to
compute it at runtime based on your analyser?

If -fstack-usage is not sufficient, understanding why might allow us to
propose a better option.

Will


Can you not use the dwarf frame unwind data?  That's always sufficient
to recover the CFA (canonical frame address - the value in SP when
executing the first instruction in a function).  It seems to me it's
unlikely you're going to need something that's an exceedingly high
performance operation.

Thank you for your comment.
Yeah, but we need some utility routines to handle unwind data(.debug_frame).
In fact, some guy has already attempted to merge (part of) libunwind into
the kernel[1], but it was rejected by the kernel community (including Linus
if I correctly remember). It seems that they thought the code was still buggy.

That is one of reasons that I wanted to implement my own analyzer.

[1] https://lkml.org/lkml/2012/2/10/129

Thanks,
-Takahiro AKASHI


R.



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